This invention relates generally to sensors and particularly to fiber optic sensors. In particular, this invention relates to fiber optic interferometric sensors arranged in arrays for measuring geophysical parameters. Still more particularly, this invention relates to fiber optic interferometric sensors that respond to perturbations such as acoustic wavefronts by producing a phase difference in two light beams propagated by fiber optic material.
Operational requirements for sensor arrays such as those used in geophysical applications include having a wide dynamic range for each individual sensor. Such sensors may require a dynamic range of about 10.sup.6 or more.
Optical fibers are sensitive to a large number of physical phenomena, such as acoustic waves and temperature fluctuations. An optical fiber exposed to such phenomena changes the amplitude, phase or polarization of light guided by the fiber. Optical fibers have been considered for use as sensing elements in devices such as hydrophones, magnetometers, accelerometers and electric current sensors.
Mach-Zehnder, Michelson, Sagnac, and resonant ring interferometers have been used as sensors. Mach-Zehnder, Michelson and Sagnac interferometers respond to the phenomenon being sensed by producing phase differences in interfering light waves. Detecting phase changes in the waves permits quantitative measurements to be made on the physical quantity being monitored. The Sagnac interferometer produces phase differences in two counter-propagating light waves in a coil of a single fiber in response to rotations about the axis of the coil.
The Mach-Zehnder interferometer is particularly sensitive to acoustic vibrations. A fiber optic Mach-Zehnder interferometer typically has a reference arm comprising a first length of optical fiber and a sensing arm comprising a second length of optical fiber. The sensing arm is exposed to a physical parameter, such as an acoustic wavefront, to be measured while the reference arm is isolated from changes in the parameter. When the Mach-Zehnder interferometer is used as an acoustic sensor, acoustic wavefronts change the optical length of the sensing arm as a function of the acoustic wave intensity. An optical coupler divides a light signal between the two arms. The signals are recombined after they have propagated through the reference and sensing arms, and the phase difference of the signals is monitored. Since the signals in the reference and sensing arms had a definite phase relation when they were introduced into the arms, changes in the phase difference are indicative of changes in the physical parameter to which the sensing arm was exposed.
A Michelson interferometer also has a sensing arm and a reference arm that propagate sensing and reference signals, respectively. However, in the Michelson interferometer these arms terminate in mirrors that cause the sensing and reference signals to traverse their respective optical paths twice before being combined to produce an interference pattern.
Arrays of acoustic sensors are used in geophysical explorations and various sonic detection applications. Previous sensor arrays commonly used in such applications include a great many active sensors, which are expensive and difficult to operate, primarily because of the large number of wires required to activate the sensors and receive data therefrom.
Large arrays formed from previous fiber optic sensors require large amounts of optical power. For example, an array of about 4000 sensors requires ten watts or more. Lasers that produce such large power outputs and that are compatible with with optical fibers are bulky, inexpensive and inefficient.